I probably don't understand aggregates.

I'm new to postgres, sql and mailing lists, so I think I'm in the right
place here.

I have a pl/pgsql function, that calls a selfwritten aggregate function
(consisting of stepfunction and finalfunction) and afterwards processes
the results.

The Problem:

if I call
SELECT myAggrFunction(x) FROM table;
I get the correct and expected output.

but when myAggrFunction gets called inside the "wrapper"-function, the
finalfunction gets executed before all rows are processed and hence I
get a "Division by zero" error.

E.g.
SELECT myWrapperFunction(x) FROM table;
ERROR: division by zero

myWrapperFunction would look something like:

BEGIN
M := myAggrFunction(x);
--do some more stuff
RETURN M;
END;

I hope the description is somehow understandable.

Any feedback will be highly appreciated.

Markus